In an electrically operated vehicle in which the power source for the vehicle is in the form of a battery powered electric motor, it is expected that the electric motor will be used as a generator for performing a regenerative braking operation to recover energy and charge the battery and use the energy for driving the motor. Because the braking force applied to the vehicle wheels through the regenerative braking operation is limited or insufficient to effect full braking, the insufficient braking force is compensated for by adding a hydraulic braking operation. Thus, it is known to perform the regenerative braking operation in combination with the hydraulic braking operation as disclosed in Japanese Unexamined Patent Publication No. Hei7-336806 published on Dec. 22, 1995.
In this brake control system, the braking operation is initiated upon depression of the brake pedal that is associated with a master cylinder. If the regenerative braking operation is set to be performed with top priority, the wheel of the electrically operated vehicle is applied with the summation of the regenerative braking torque and the wheel brake cylinder pressure from an electric motor and the wheel brake master cylinder, respectively. Under such a regenerative braking operation priority mode, as can be seen from FIG. 4(a), the wheel brake cylinder pressure is in the form of a reduced value of the static pressure (master cylinder pressure) from the master cylinder connected to the wheel brake cylinder. The reason why the master cylinder pressure is not supplied, as is, to the wheel brake cylinder is that as can be seen from FIG.4(a), until the regenerative braking torque reaches its maximum value or about 0.2 G, no additional braking force is required to the wheel brake cylinder from the master cylinder for being added to the regenerative braking torque. If the master cylinder pressure exceeds 0.2 G due to further brake pedal depression, a real supply of the master cylinder pressure to the wheel brake cylinder begins. The master cylinder pressure supplied to the wheel brake cylinder, which is regulated by a set of a pressure reducing valves, results in a wheel cylinder of FIG. 4(a). FIG. 4(b) represents the relationship between the braking force applied to the wheel and the master cylinder pressure.
Sometimes, a bad braking operation is felt due to an excessive depression of the brake pedal which corresponds to the pressure to be supplied to the wheel brake cylinder. To prevent such a phenomena, when the regenerative braking torque reaches its maximum value, a dynamic hydraulic pressure is supplied to the wheel brake cylinder from a dynamic hydraulic pressure generating device for compensating for the deficient braking force. This supply is established by fluidly isolating the master cylinder from the wheel brake cylinder. The dynamic hydraulic pressure device includes a reservoir storing an amount of brake fluid, a pump and a regulator. The regulator is associated with the master cylinder, thereby constituting a hydraulic-booster as is well known and the brake fluid under pressure fed to the regulator from the reservoir while the pump is being driven is set to be equalized to the master cylinder pressure in response to the brake pedal depression. The resultant dynamic pressure is supplied to the wheel brake cylinder.
As shown in FIG. 5(a), in response to the depression of the brake pedal, the master cylinder pressure increases. On the other hand, as shown in FIG. 5(b), the regenerative braking force depicted in solid line has the maximum value as a limit value and sometimes becomes less than the ideal braking force depicted in dotted line and corresponding to the master cylinder pressure. Thus, when a signal shown in FIG. 5(c) is generated, a valve disposed between the regulator and the wheel brake cylinder is opened for a duration of time to supply the hydraulic pressure to the wheel from the regulator. Thus, as shown in FIG. 5(d), a composition of the regenerative braking force and the resultant hydraulic pressure becomes substantially equal to the ideal or required braking force to be applied to the wheel cylinder.
However, the foregoing supply of the hydraulic pressure to the wheel brake cylinder is established by isolating the master cylinder from the wheel brake cylinder, with the result that despite the driver's depression of the brake pedal little stroke or movement thereof is established. Thus, the driver feels a rigidity in the brake pedal which is a reverse phenomena to the so-called spongy or soft brake feel.
A need therefore exists for a brake control system for an electrically operated vehicle which is free from the foregoing disadvantage and drawback.
It would also be desirable to provide a brake control system for an electrically operated vehicle in which the brake pedal can be depressed without establishing a rigid feeling of the brake pedal.